1. Field of the Invention
The present invention relates in general to optical devices, and relates in particular to a white light source which generates a coherent super-wideband white light on both ends of a pump optical pulse wavelength, and to optical devices utilizing the coherent white light source.
2. Description of the Related Art
A conventional method for generating super-wideband white light is to excite various nonlinear optical materials with ultra-short light pulses in the range of pico to femto seconds and to generate a combined effect of third order nonlinear effects so as to achieve a huge increase in the bandwidth of the generated spectrum. Nonlinear optical materials include such gaseous substances as xenon, liquid substances such as CS.sub.2, D.sub.2 O, CCl.sub.4, and solid substances such as glass, optical fiber, and semiconductors.
The spectral pattern of the white light generated in a multi-mode optical fiber as a nonlinear optical material is discontinuous, as shown in FIG. 23, and it can be seen that the spectral components are very complex. The spectral power of the generated white light is not uniform nor constant over the range of the generated wavelengths. Furthermore, it is necessary to use a pump power of over 100 W, necessitating the use of a high power large-size laser sources (a pulse repetition frequency of which is in the order of 100 MHz), such as a solid state laser, as a pump light source. For this reason, conventional white light could not be used as a light source for optical communications which require a repetition frequency in excess of giga hertz. (Refer to R. R. Alfano Ed., "The Supercontinum Laser Source", Springer-Verlag, 1989, New York.)
Furthermore, because the generated white light pulses lack coherency; even if certain wavelength components are filtered out from the white light spectrum with a wavelength selective device such as an optical bandpass filter, it is not possible to obtain a transform-limited pulse (referred to as a TL-pulse) having the minimum pulse width for a given bandwidth which is determined by the time-frequency relationships in the Fourier transform. More specifically, with reference to R. Dorsinvilie, et al., "Generation of 3-ps pulses by spectral selection of the continuum generated by a 25-ps harmonic Nd:YAG laser pulse in a liquid", Applied Opt., 27, pp.16-18, 1988; and M. N. Islam, et al., "Broad bandwidths from frequency-shifting solitons in fibers", Opt. Lett., 14, pp. 370-372, 1989, the time-bandwidth product (a product of the pulse width and the spectral bandwidth) of the generated white light pulses are several times or up to ten times larger compared with the minimum value which is given by the corresponding time-bandwidth product of the TL-pulse. This means that to generate a given pulsewidth, it is necessary to have a bandwidth up to ten times wider, and it is difficult to utilize the generated white pulses from such a source in optical fiber communication systems which are affected by the dispersion effects in the optical fibers. Moreover, those low-coherent white light sources produce a large amount of beat noise, meaning that it is difficult to carry a signal on them.
The reasons that the white pulses produced by the conventional method exhibit a complex spectrum, that the light source requires a high excitation power, that the resulting white light lacks coherency are due to the fact that the third-order nonlinear optical effects are achieved primarily by a combined effect of stimulated Raman scattering, self-phase modulation, cross-phase modulation and the four-wave mixing process involving phase-matching of higher order transverse spatial-mode waves.
The mechanism of the conventional method of white pulse generation is explained in more detail below. First, Raman light is generated by stimulated Raman scattering, with its center wavelength shifted to the longer wavelength side by an amount equal to the Raman shift characteristic of the optical material. Or, a multiple of four-wave mixing light components are generated, with a center wavelength centered about the pump light wavelength by the process of phase matching in higher order transverse-modes. When the pump power is increased further, the newly generated spectral components will grow to their full extent, and are spread out further by the processes of self-phase modulation, cross-phase modulation. The result is that the line spectra of the pump light and these component waves superimpose on each other to generate a white-light band. The white light spectrum thus generated exhibits a complex structure, and the necessary pump power is high because the stimulated Raman scattering, four-wave mixing, self-phase modulation and cross-phase modulation are all involved. The coherency of the white light spectrum becomes degraded because the spectrum is affected by the complex phase modulations involving the self-phase and cross-phase modulations in the generation process.
It results that the conventional method of generating white light cannot provide white pulses of high coherency having a continuous and uniform spectrum, and its application has been limited to special uses in laboratories, such as optical spectroscopy sources pumped by high power lasers.
On the other hand, the future optical communication is expected to be based on a system combining the techniques of optical time-division multiplexing (optical TDM) and wavelength-division multiplexing (WDM) to a achieve a quantum jump in the transmission capacity of optical fibers. The optical TDM technique is based on multiplexing optical pulses from different channels on a timescale, and the transmission capacity is increased by the additional number of channels. The WDM technique is based on superimposing a signal wave on optical carrier frequencies (wavelengths) of many different wavelengths, and the transmission capacity is increased by the additional number of optical carrier waves.
In the past, to obtain different optical carrier frequencies needed to operate the WDM technique, it has been proposed to simultaneously select several wavelength components by filtering white light having a super-wide band through optical filters. The optical carrier frequencies thus obtained are significantly more controllable in terms of wavelength and its temperature stability than those produced by using a number of different light sources, depending merely on the properties of the optical filters employed. However, because the white light previously produced by the conventional method lacks coherency, as explained above, it was difficult to generate multi-wavelength wideband signals in the giga Hz repetition region having a high signal to noise (S/N) ratio and to further time-division multiplex such signals to obtain low-noise TDM/WDM multiplexed signals. Therefore, in the conventional WDM technique, it was necessary to provide as many sources of light as the number of carrier waves that are necessary for that particular communication system.
The device configuration for generating wavelength-division multiplexed signals in the conventional WDM technique is shown in FIG. 24.
The device comprises: a plurality (n pieces in the FIG. 24) of laser sources 101-1 to 101-n oscillating at different optical frequencies to generate a plurality of wavelengths; and the corresponding external modulators 102-1 to 102-n; where each of the output optical signal is modulated electrically in an assigned channel. The signals are combined in an optical multiplexer 103 and are forwarded to an optical transmission route 104.
In the process, a portion of the output light from the optical multiplexer 103 is used to control the oscillation optical frequencies of the individual laser sources so that the output is spaced apart periodically. In practice, the oscillation optical frequency of each laser source is matched to the transmission frequency of a periodic optical filter 105, such as a ring resonator having a property to produce periodic transmissions. This operation of the filter 105 is as follows. Each laser source 101-1 to 101-n is modulated by the low-frequency electrical signal F.sub.1 to F.sub.n output from the respective oscillators 106-1 to 106-n. A portion of the output light from the optical multiplexer 103 passes through the periodic optical filter 105 and is converted to electrical signals by the optical/electrical conversion circuit 107, and are transformed by the low frequency electrical signals F.sub.1 to F.sub.n into respective baseband signals for each channel. Each of the baseband signals, after passing through the low-pass filter (LPF) 108-1 to 108-n and the proportional differentiation/integration circuitries (PID) 109-1 to 109-n, is made to feedback into the corresponding biasing electrical current for each of the laser sources, and are controlled so as to match the oscillation frequencies of laser sources 101-1 to 101-n with the transmission frequency of the periodic optical filter 105.
As described above, although the device is controlled to make the oscillation frequencies of each of the laser sources 101-1 to 101-n to be periodically spaced, in the conventional wavelength-division multiplexed signal generation device, it is not easy to control the device so as to produce an absolute wavelength to serve as a reference wavelength.
The need for a super-wideband, coherent white light source having a uniform and continuous spectrum is not limited to applications in a WDM-based system. From viewpoints of simplifying the construction and economy of the apparatus, such a light source is needed in the field of measuring devices to study various wavelength-dependent optical properties, such as delay-times for a group of optical devices, and to define time-response characteristics of nonlinear phenomena.